Style <select> element based on selected <option>

Impossible? Hold my beer.

Make the select element aware of the current option. CSS can handle the rest. For this, all we need is a value assignment in the onchange event. With a little more effort you can also initialize the <select> tag by its original value; not included below but it's super easy, just give it a "data-chosen" property when composing the markup.

    <select onchange=" this.dataset.chosen = this.value; ">
        ...
        ...
    </select>

And now you can easily target & style it:

select[data-chosen='opt3'] { 
    border: 2px solid red;
}

See on CodePen.

Side note: "chosen" is no magic word, just a descriptive name - it could be "SantaClaus". It would also look better that way.

Unfortunately, yes - this is something not currently possible with only CSS. As mentioned in the answers and comments to this question, there is currently no way to make the parent element receive styling based on its children.

In order to do what you're wanting, you would essentially have to detect which of the children (<option>) is selected, and then style the parent accordingly.

You could, however, accomplish this with a very simple jQuery call, as follows:

HTML

<select>
  <option value="foo">Foo!</option>
  <option value="bar">Bar!</option>
</select>

jQuery

var $select = $('select');
$select.each(function() {
    $(this).addClass($(this).children(':selected').val());
}).on('change', function(ev) {
    $(this).attr('class', '').addClass($(this).children(':selected').val());
});

CSS

select, option { background: #fff; }
select.foo, option[value="foo"] { background: red; }
select.bar, option[value="bar"] { background: green; }

Here is a working jsFiddle.

Back to the question about the future of selectors. Yes - the "Subject" selectors are intended to do exactly what you mention. If/when they ever actually go live in modern browsers, you could adapt the above code to:

select { background: #fff; }
!select > option[value="foo"]:checked { background: red; }
!select > option[value="bar"]:checked { background: green; }

As a side-note, there is still debate about whether the ! should go before or after the subject. This is based on the programming standard of !something meaning "not something". As a result, the subject-based CSS might actually wind up looking like this instead:

select { background: #fff; }
select! > option[value="foo"]:checked { background: red; }
select! > option[value="bar"]:checked { background: green; }